This invention relates to process for the electroplating of aluminum parts, including the electroplating of coinage blanks. The invention also extends to electroplated aluminum parts, including coinage products.
Electroplating of aluminum or aluminum alloy substrates is more difficult than on many other materials because an oxide film coats aluminum immediately when exposed to air or water. This oxide film results in uneven deposition of electroplates, and poor adhesion of the plate. Several approaches exist for the pretreatment of aluminum and aluminum alloys substrates for electroplating. Those include a) etching, in which the substrate is pitted with an attacking solution, b) anodizing, in which an oxide film is thickened by anodizing and then etched to roughen the surface; c) electroless nickel plating, in which nickel is deposited from solution without the use of an applied current, and d) precoating, in which the oxide film is first removed with cleaners or acid, and then immediately coated with tin or zinc, more typically zinc, by immersion deposition. When zinc is used, this precoating process is known as zincating, the immersion solution is termed a zincate or zincating solution, and the coating is often termed a zincate coating or zincate layer.
Kodak developed and patented zincating solutions in about 1927. It was a simple solution of sodium hydroxide and zinc chloride. Later, in 1953, W. G. Zelley proposed three zincating solutions that are referred to as xe2x80x9csimplexe2x80x9d zincating solutions. The three xe2x80x9csimplexe2x80x9d zincating solutions, together with typical substrate cleaning, conditioning, and post-zincating strike layers, are discussed in ASTM B253-87 xe2x80x9cPreparation of Aluminum Alloys for Electroplating.xe2x80x9d The drawbacks of the simple zincating solutions were that they had to be operated differently for different aluminum alloys and that the adhesion of the electroplated layer to the aluminum was inconsistent. Subsequent improvements to zincating aluminum included using zincate solutions containing elements such as copper, nickel and iron, with complexing agents such as cyanide and tartrate, to keep the metals in solution, and double dipping in which a first zincate immersion coating was stripped off in a suitable acid prior to forming a second zincate immersion coating.
In the 1960""s, W. Canning Ltd. developed a Modified Alloy Zincate (MAZ) solution. This solution was designed to generate improved adhesion over the simple zincating solutions, to eliminate the need for depositing intermediate strike layers of metals such as copper, brass or nickel prior to electroplating, and to produce more consistent process results. Included in the preferred MAZ solution besides zinc, were the additional metals of copper, nickel and iron. This work is referenced in Great Britain Patent 1,007,252, granted in 1965.
In spite of many advances made in the electroplating of aluminum and its alloys, adhesion of the electroplate to the substrate still continues to be a problem. While a weakly adherent electroplated layer may suffice for applications in which the final product is primarily aesthetic, many practical applications demand good adhesion of the electroplated layers to the underlying aluminum substrate.
A particularly difficult environment for electroplated products is circulation coinage. Today, many countries of the world rely on plated coinage in which coinage metals, such as nickel, copper, bronze or brass overlayers are electroplated onto cores of coinage metals such as zinc, steel, or nickel. Processes of electroplating such coinage cores have been developed to ensure that a highly-adherent electroplated layer is formed which can withstand a bend test. The bend test is one indication of whether the plated coinage product can withstand the rigors of a deforming process, that is a minting step, without delamination of the electroplated layers from the substrate. While bend tests may vary, in general, to pass such a test for circulation coinage, the plated coin blank is bent through a 90xc2x0 angle and the plated layer must not be removable with a sharp instrument such as a file or knife. Although aluminum and its alloys have been used in coins, to the inventors"" knowledge, no electroplated circulation coinage products with aluminum or aluminum alloy cores exist in the world today. Efforts by the inventors to apply a simple zincating solution, or an MAZ solution to aluminum substrates, as set out in the Examples of this application, failed to produce adequate adhesion to pass a bend test.
Japanese Patent Application JP 19910146184, published as JP 4369793 on Dec. 22, 1992, to Yagiken, K K and others, describes gaming tokens produced from aluminum or its alloys to include a colored anodized layer and zinc nickeling or zinc-nickel-chrome plating. Japanese Patent Application JP19910187628, published as JP 535963 on Feb. 12, 1993, to Yagiken, K K and others, also discusses game machine tokens and their manufacture. This latter reference uses a zincating procedure to coat aluminum blanks that are used for game machines. The zincate referred to in this patent is Substar(trademark) ZN-111 manufactured by Okuno Reagent Industry of Japan. There is no indication in the reference that the tokens are minted after plating. Efforts by the inventors to duplicate the process of this Japanese reference, as set out in Example 8 of this application, failed to produce a coinage product with sufficient adhesion of the plate to function as a circulation coin.
There is still a need for an effective aluminum pretreatment process for the electroplating of aluminum parts, which results in a plate with sufficient adhesion to withstand the rigors of a deformation process. There is a particular need, for coinage purposes, of an aluminum pretreatment and electroplating process which will produce a plated coinage product which can withstand a bend test without causing delamination of the electroplated layers from the underlying substrate.
The present invention provides both an improved zincating and an improved copper strike process for the pretreatment of substrates of aluminum and its alloys, such that subsequent electroplating layers are sufficiently adherent so as to withstand a deformation process without causing delamination of the electroplated layers from the substrate. In a preferred embodiment, the pretreatment processes of this invention are capable of producing electroplated products which meet the rigorous adhesion requirements of the circulation coinage industry and allow for the mass production of small barrel electroplated parts such as coinage blanks. The process has been demonstrated to produce electroplated coin blanks with very good adhesion of several different electroplated layers to the aluminum substrate, and to allow a strike of the zincated aluminum coin blanks at practical current densities for barrel electroplating.
The improved copper strike process of this invention has the advantage of operating at realistic and efficient current densities for barrel plating. Standard electroplating barrels are limited to currents of about 1000 Amps, and a typical operational current density in manufacturing is approximately 0.25 A/dm2, based on total area of the charge. The literature relating to plating aluminum refers to current densities from 2.5 A/dm2-40 A/dm2. As the standard electroplating barrel establishes a total current limitation of about 1000 Amps, the only method of increasing the current density is by reducing the area of the quantity of parts that are in the barrel. Reducing the loading of the barrel translates into a loss of manufacturing productivity in barrel electroplating.
In developing the process of this invention, the inventors determined that simple zincating solutions, together wilt those developed as MAZ and Substar(trademark) (as referred to above), were inadequate to meet the manufacturing and quality requirements for electroplated coinage. In particular these prior art zincating processes did not produce a plated coin blank which could withstand a bend test, which is a standard known in the coinage industry. The first attempt at producing barrel electroplated aluminum coinage was a zincating solution composed of the following components: 500 gpl NaOH, 100 gpl ZnO, and 2 gpl FeCl3 (see Example 6). The blanks were coated with zinc using a two step zincating process. Following a copper strike and electroplating, the blanks were subjected to the bend test, and according to the ASTM bend test standard, the blanks failed the test. The coating cracked along the bend, and it was possible to peel off the coating with the fingers.
As an alternative, a more dilute simple zincate bath was tested by the inventors, and the electroplated aluminum blanks exhibited similarly poor results in the bend test. This zincating solution had a composition of 100 gpl NaOH, 20 gpl ZnO, and 2 gpl FeCl3. The aluminum blanks were zincated in a two step zincating process, placed in a standard high current density copper strike bath, and then electroplated in standard copper cyanide electroplating solution. After this process, individual blanks were bent to check the adhesion of the coating to the aluminum. It was possible to remove the coating with the fingers following this test.
In another attempt to improve the adhesion of the electroplated layer, the inventors tried a Modified Alloy Zincate (MAZ) solution from British Patent 1,007,252 (see Example 7). This zincating bath had a composition of NaOH of 106 gpl, zinc sulfate 40 gpl, nickel sulfate hexahydrate 30 gpl, zinc sulfate heptahydrate 40 gpl, potassium hydrogen tartrate 50 gpl, and copper sulfate pentahydrate. The adhesion of the subsequent electroplate, even when a copper strike was included, was not adequate for circulation purposes because following the bend test it was still possible to remove the electroplated coating using a sharp instrument.
Early work by the inventors established that bettor adhesion of the electroplated coating to the aluminum, as demonstrated by a standard bend test, and the most consistent results, were achieved by using both the improved zincating process and the copper strike process developed by the inventors. A two-step zincate process was used in which the composition of the zincate bath was 273 gpl NaOH, 24 gpl NiSO46H2O, 8.7 gpl CuSO45H2O, 40 gpl ZnSO4, 40 gpl ZnSO47H2O, 1.7 gpl iron chloride, and a complexing agent to keep the ions in solution. The copper strike had a free cyanide composition of 15 gpl, the copper cyanide was 30 gpl, and the pH was 8.5. The copper strike could be operated at a wide variety of current densities ranging from 0.10 A/dm2 and upwards. After the bend test, the coating was still very strongly adhered to the blanks and it was not possible to remove the coating using a sharp instrument.
Later work by the inventors established that a higher hydroxide amount in the zincating step, for example about 136 gpl hydroxide (about 320 gpl NaOH), was more preferred, allowing the zincating step to be conducted closer to ambient temperature. The inventors further discovered that the addition of up to about 10 gpl KCN in the zincating step, as a complexing agent and a solution activator, improved adhesion. Furthermore, the is inventors established that the copper strike could be conducted at higher pH, in the range of about 8.5-11.0, with a lower free cyanide range of about 8.0-12.0 gpl, at an elevated temperature of about 40-45xc2x0 C., to achieve excellent adhesion.
In one broad aspect, the invention provides an improvement in a process for electroplating aluminum parts or aluminum strip, in which the aluminum part or strip is pretreated with a zincate solution containing the ions of hydroxide, zinc, nickel and copper, In accordance with the present invention, the improvement comprises providing the zincate solution so as to produce hydroxide ions in an amount in the range of 75-175 gpl, zinc ions in an amount in the range of 15-40 gpl, nickel ions in an amount in the range of 2-10 gpl and copper ions in an amount in the range of 1.5-5 gpl. Most preferably, the improved process also includes applying a strike layer of a coinage metal, preferably copper or nickel, to the aluminum part or strip after zincating. Most preferably the strike layer is copper, applied from a copper cyanide strike bath at a pH in the range of 8.5-11.0 (more preferably 9-10.5), with a free cyanide range of about 3.0-35.0 gpl (more preferably 8.0-12.0), and a temperature of about 40-45xc2x0 C., using a current density in the range of 0.1-10 A/dm2.
In another broad aspect, the invention provides a method of electroplating pre-cleaned aluminum parts, comprising:
a) loading the pre-cleaned aluminum parts into a perforated electroplating barrel;
b) immersing the barrel into a zincate solution to submerge the aluminum parts, and tumbling the aluminum parts in the barrel to form a first zincate layer on the aluminum parts, the zincate solution containing hydroxide ions in an amount in the range of 75-175 gpl, zinc ions in an amount in the range of 15-40 gpl, nickel ions in an amount in the range of 2-10 gpl, and copper ions in an amount in the range of 1.5-5 gpl;
c) immersing the barrel into an acid solution to submerge the aluminum parts and to strip the first zincate layer;
d) immersing the barrel in a zincate solution having a composition as set forth in step (b), to submerge the aluminum parts, and tumbling the aluminum parts in the barrel to form a second zincate layer which completely covers the aluminum parts;
e) immersing the barrel in a strike bath of a strike metal, to submerge the aluminum parts, and tumbling the aluminum parts in the barrel while applying an electrical current to the aluminum parts in the barrel, to apply a strike layer of the strike metal to the aluminum parts;
f) immersing the barrel in one or more electroplating baths of one or more metals, to submerge the aluminum parts, and tumbling the aluminum parts in the barrel while applying an electrical current to the aluminum parts in the barrel, to apply one or more electroplated layers of the one or more metals or of an alloy of the metals to the aluminum parts; and
g) removing the electroplated aluminum parts from the barrel.
In yet another broad aspect, the invention provides an electroplated aluminum part or strip, comprising:
a substrate formed from aluminum or an aluminum alloy and having multiple surfaces;
a layer of zincate on at least one of the surfaces of the substrate;
a strike layer of a strike metal covering the layer of zincate; and
one or more electroplated layers of one or more metals covering the strike layer, said one or more electroplated layers adhering to the substrate to withstand a deformation process without delamination from the substrate.
In preferred embodiments, the electroplated aluminum parts of this invention comprise electroplated coin blanks, in which the strike metal is preferably copper or nickel, most preferably copper, and wherein the one or more electroplated layers is composed of one or more of coinage metals or alloys, preferably selected to provide one or more electroplated layers (bright or matte) of one or more of nickel, copper, bronze, brass, silver, gold, platinum and alloys thereof. The electroplated coin blanks of this invention have been proven to provide adhesion of the electroplated layer(s) to the substrate sufficient to withstand a minting step, or a bend test, making them suitable for circulation coinage.
By xe2x80x9cstrike metalxe2x80x9d as used herein and in the claims is meant any metals capable of being plated by electroplating or electroless plating to provide a thin adherent layer of the metal.
By xe2x80x9cdeformationxe2x80x9d as used herein and in the claims is meant plastic deformation of a metal, in which the volume and mass of the metal are conserved and the metal is displaced from one location to another. Deformation processes include forging, rolling, wire drawing, extrusion, deep drawing, stretch forming, bending, and shearing. Minting is an example of a forging step.
By xe2x80x9cmintablexe2x80x9d as used herein and in the claims is meant that a coinage blank has the following characteristics: sufficiently soft to take an impression on striking (generally about 0.02 mm to 5 mm relief detail with practical loads on commercial minting presses); having an electroplate with a fine grain size to permit complete filling of the minting die and uniform metal flow; having a controlled surface finish after minting, such as frosted, glossy and/or matte; and having friction and flow characteristics in the minting dies such that acceptably long minting die lives can be obtained.